It has been known for decades that the hippocampal region plays a prominent role in learning and memory, particularly in the organization of memories and in their permanent consolidation. Yet, despite significant advances in characterizing hippocampal-dependent memory, we have little insight into the nature of the neural coding that underlies memory representation by the hippocampus, or into the nature of cortical- hippocampal interactions that likely mediate the consolidation processes. The long-term objective of this project is to clarify the hippocampal "memory code" by analyzing firing patterns of single neurons and neuronal ensembles of cortical and hippocampal areas in rats performing behavioral tasks that have different cognitive and memory demands, and to identify aspects of cortical coding dependent on components of the hippocampal system. A major technical thrust of the proposed studies involves the development of new neural analysis techniques. These techniques will be applied to characterize spike trains of individual cells and population activity patterns of neuronal ensembles that distinguish sensory, behavioral, and cognitive events. We will determine the extent to which neural activity is controlled by the sensory qualities of stimuli and by abstract relations among sensory cues, whether firing is best time-locked to sensory or behavioral events, how long encodings persist after brief stimulus exposures, and when they appear during the course of extended training. The major theoretical thrust of this research program is aimed at testing a working hypothesis about stages of memory processing and interactions among brain structures across stages. The olfactory cortex, parahippocampal region, and hippocampus compose a closely and reciprocally interconnected network that will be used to study how these structures interact to mediate different forms of memory. Experiments will be aimed at testing specific predictions of the model, including: (1) that the olfactory cortex supports short-term and permanent memory by alterations in sensory 'tuning', (2) that the parahippocampal region maintains representations of individual and configural cues for an intermediate period, (3) that the hippocampus encodes relationships among items but, in at least some situations, does not store information about specific items, and (4) that components of the hippocampal system mediate associations between individual stimulus representations in the cortex. By characterizing and comparing the nature of coding properties in different components of this system, we will improve our understanding of the functions performed by the hippocampus within the larger system of cortical structures in which it operates.